Chromatography columns packed with microparticulate sorbents have been widely applied as separation media. Despite many advantages, such microparticulate sorbents have limitations such as large void volume, slow mass diffusion, and the need for end frits to support and retain the microparticulate sorbents, which results in a non-homogeneous column. Monolithic chromatography beds are proposed to alleviate the problem of restricted mass transfer and eliminate the need for end frits.
Monolithic chromatography beds are porous, uniform blocks taking the form of the containment volume. Generally, monolithic chromatography beds are classified as organic polymer-based or silica-based materials. Most organic polymer-based beds are prepared by radical polymerization such as polymerization of methacrylates and poly(styrene-co-divinylbenzene). In these instances a fused silica capillary or column is filled with a polymerization mixture and radical polymerization initiated through heating. When the polymerization is complete, unreacted components may be removed physically or by electrosomotic flow. The porosity of the polymer can be partially controlled by the degree of cross linking and the amount of organic solvent used as pores are formed by the removal of the organic solvent.
Silica-based monolithic beds are typically prepared using an acid-catalyzed sol-gel process, which involves hydrolysis, condensation reactions and polycondensation of silica alkoxide precursors. Commonly used precursors are tetraethoxysilanes(TEOS) and tetramethoxysilanes (TMOS). Preparation of the beds involves mixing the precursors prior to introduction into a capillary column and a progenic reagent is added such as polyethylene glycol or poly(ethylene) oxide. The ratio of progenic reagent and silica precursors controls pore size.
Monolithic chromatography beds are used in bioanalytical assays due to high rates of mass transfer and permeability of the pores to large molecules. Thus separation of proteins, oligonucleotides, DNA, virus, bacteria or microorganisms can be achieved. Immunoaffinity separations based on antigen-antibody reactions have also been achieved. Columns based on methacryate-co-ethylene dimethacraylate (GMA-EDMA) are commercially available which contain chemically reactive epoxy groups, which can react with amino groups contained in the biological agent.
The possibility of ROMP-based (ring opening metathesis polymerization). monolithic materials and the corresponding separation efficiency in protein separation have been demonstrated. However, the ROMP-based monolithic material uses organic solvents such as isopropanol. The use of organic solvents is costly and adds to environmental concerns. Therefore the development of aqueous ROMP-based monolith chromatography media is desirable but has been limited by the lack of a highly reactive water-soluble ROMP catalyst.